Unconventional coordination modes of carboranyl phosphine–iminophosphorane ligands

A series of new carboranyl phosphine-iminophosphorane ligands were obtained and characterized. The ligands present a flexible three-atom spacer between (P,N) donor atoms, derived from the diphosphine <i>dppe</i>. The ligands include examples of C-carboranyl derivatives (ligands L1-L3) and B3-carboranyl derivatives (ligands L4 and L5), to compare the different effect of these electron-withdrawing groups. An organic analog was also prepared (ligand L6) for comparison purposes. The analysis of the structures of their dichloride palladium(II) complexes using several techniques, including X-ray diffraction, reveals that the carboranyl group can affect the coordinating ability of the attached nitrogen atom. Thus, although organic phosphine-iminophosphorane ligands always show the (P,N) chelating mode, providing both sites are available, the new carboranyl ligands give rise to a surprising variety of coordination modes: the expected (P,N) chelating mode, the unusual P-terminal mode and the unprecedented (P,N) bridging mode. The comparison of these structures with organic analogues and with the carboranyl ligands derived from the diphosphine <i>dppm</i> indicates that the length of the spacer between the donor atoms is an important variable for the coordination mode of the ligand, once the coordinating strength of the nitrogen atom is reduced by effect of the carborane cage.